A Dialogue of Learning: The Exploration of a Service-Learning Practicum and the Development of Democratic Educational Values

Using a hybrid portraiture interpretivist case study methodology, this study explores the development of democratic educational values of pre-service teachers who participated in a “nested” service-learning practicum during their first semester in a secondary teacher preparation program. In this nested model, both the pre-service teachers and the middle school students with whom they worked participated in service-learning. The study is in response to the findings of previous researchers that democratic educational values have, in many classrooms, been pushed aside by the pressures of the standardization and accountability movement and by the belief that democratic educational values are critical to a public educational system which supports civic identity and participation. Data collected over the course of one semester included reflective journals, blog postings, observations of the service-learning seminar, observations of teaching practices in the field, and audio-recorded semi-structured interviews. Four participants were interviewed three times each, and all four participants were observed both in the service-learning seminar and in their field placements. While this study did not find that participation in a nested service-learning model led to pre-service teachers becoming active agents of change, it did find that the nested service-learning experience helped the pre-service teachers to begin to lay a solid foundation in their understanding of basic democratic educational values, in their plans to embrace democratic educational values in their future classrooms, and in their view of themselves as democratic educators

The Diatom Flora of the Red Lake Peatland, Minnesota

Diatoms collected from three transects in the Red Lake Peatland occur in characteristic assemblages. One hundred two taxa were observed from 26 sample sites, with Eunotia exigua and Pinnularia rupestris the most dominant species. Clustering indicated three peatland types were present: rich fen, transitional and poor fen, and bog

Gold fragmentation induced by stopped antiprotons

A natural gold target was irradiated with the antiproton beam from the Low Energy Antiproton Ring at CERN. Antiprotons of 200 MeV/c momentum were stopped in a thick target, products of their annihilations on Au nuclei were detected using the off-line gamma-ray spectroscopy method. In total, yields for 114 residual nuclei were determined, providing a data set to deduce the complete mass and charge distribution of all products with A > 20 from a fitting procedure. The contribution of evaporation and fission decay modes to the total reaction cross section as well as the mean mass loss were estimated. The fission probability for Au absorbing antiprotons at rest was determined to be equal to (3.8+-0.5)%, in good agreement with an estimation derived using other techniques. The mass-charge yield distribution was compared with the results obtained for proton and pion induced gold fragmentation. On the average, the energy released in pbar annihilation is similar to that introduced by ~ 1 GeV protons. However, compared to proton bombardment products, the yield distribution of antiproton absorption residues in the N-Z plane is clearly distinct. The data for antiprotons exhibit also a substantial influence of odd-even and shell effects.Comment: 14 pages, 9 figures, Revtex 4, to be published in Physical Review

Information on antiprotonic atoms and the nuclear periphery from the PS209 experiment

In the PS209 experiments at CERN two kinds of measurements were performed: the in-beam measurement of X-rays from antiprotonic atoms and the radiochemical, off-line determination of the yield of annihilation products with mass number A_t -1 (less by 1 than the target mass). Both methods give observables which allows to study the peripheral matter density composition and distribution.Comment: LaTeX (espcrc1 style), 6 pages, 3 EPS figures, 1 table, Proceedings of the Sixth Biennal Conference on Low-Energy Antiproton Physics LEAP 2000, Venice, Ital

Neutron density distributions from antiprotonic 208Pb and 209Bi atoms

The X-ray cascade from antiprotonic atoms was studied for 208Pb and 209Bi. Widths and shifts of the levels due to the strong interaction were determined. Using modern antiproton-nucleus optical potentials the neutron densities in the nuclear periphery were deduced. Assuming two parameter Fermi distributions (2pF) describing the proton and neutron densities the neutron rms radii were deduced for both nuclei. The difference of neutron and proton rms radii /\r_np equal to 0.16 +-(0.02)_{stat} +- (0.04)_{syst} fm for 208Pb and 0.14 +- (0.04)_{stat} +- (0.04)_{syst} fm for 209Bi were determined and the assigned systematic errors are discussed. The /\r_np values and the deduced shapes of the neutron distributions are compared with mean field model calculations.Comment: 22 pages, 8 tables, 15 figure

Optimal search strategies for hidden targets

What is the fastest way of finding a randomly hidden target? This question of general relevance is of vital importance for foraging animals. Experimental observations reveal that the search behaviour of foragers is generally intermittent: active search phases randomly alternate with phases of fast ballistic motion. In this letter, we study the efficiency of this type of two states search strategies, by calculating analytically the mean first passage time at the target. We model the perception mecanism involved in the active search phase by a diffusive process. In this framework, we show that the search strategy is optimal when the average duration of "motion phases" varies like the power either 3/5 or 2/3 of the average duration of "search phases", depending on the regime. This scaling accounts for experimental data over a wide range of species, which suggests that the kinetics of search trajectories is a determining factor optimized by foragers and that the perception activity is adequately described by a diffusion process.Comment: 4 pages, 5 figures. to appear in Phys. Rev. Let

Tracking the phase-transition energy in disassembly of hot nuclei

In efforts to determine phase transitions in the disintegration of highly excited heavy nuclei, a popular practice is to parametrise the yields of isotopes as a function of temperature in the form $Y(z)=z^{-\tau}f(z^{\sigma}(T-T_0))$, where $Y(z)$'s are the measured yields and $\tau, \sigma$ and $T_0$ are fitted to the yields. Here $T_0$ would be interpreted as the phase transition temperature. For finite systems such as those obtained in nuclear collisions, this parametrisation is only approximate and hence allows for extraction of $T_0$ in more than one way. In this work we look in detail at how values of $T_0$ differ, depending on methods of extraction. It should be mentioned that for finite systems, this approximate parametrisation works not only at the critical point, but also for first order phase transitions (at least in some models). Thus the approximate fit is no guarantee that one is seeing a critical phenomenon. A different but more conventional search for the nuclear phase transition would look for a maximum in the specific heat as a function of temperature $T_2$. In this case $T_2$ is interpreted as the phase transition temperature. Ideally $T_0$ and $T_2$ would coincide. We invesigate this possibility, both in theory and from the ISiS data, performing both canonical ($T$) and microcanonical ($e=E^*/A$) calculations. Although more than one value of $T_0$ can be extracted from the approximate parmetrisation, the work here points to the best value from among the choices. Several interesting results, seen in theoretical calculations, are borne out in experiment.Comment: Revtex, 10 pages including 8 figures and 2 table

Thermal excitation of heavy nuclei with 5-15 GeV/c antiproton, proton and pion beams

Excitation-energy distributions have been derived from measurements of 5.0-14.6 GeV/c antiproton, proton and pion reactions with $^{197}$Au target nuclei, using the ISiS 4$\pi$ detector array. The maximum probability for producing high excitation-energy events is found for the antiproton beam relative to other hadrons, $^3$He and $\bar{p}$ beams from LEAR. For protons and pions, the excitation-energy distributions are nearly independent of hadron type and beam momentum above about 8 GeV/c. The excitation energy enhancement for $\bar{p}$ beams and the saturation effect are qualitatively consistent with intranuclear cascade code predictions. For all systems studied, maximum cluster sizes are observed for residues with E*/A $\sim$ 6 MeV.Comment: 14 pages including 5 figures and 1 table. Accepted in Physics Letter B. also available at http://nuchem.iucf.indiana.edu

Strong interaction and E2 effect in even- A antiprotonic Te atoms

The x-ray cascade from antiprotonic atoms was studied for Te-122, Te-124, Te-126, Te-128, and Te-130. Widths and shifts due to the strong interaction were deduced for several levels. The E2 nuclear resonance effect was observed in all investigated nuclei. In Te-130 the E2 resonance allowed to determine level widths and shifts of the LS-split deeply bound (n,l)=(6,5) state, otherwise unobservable. The measured level widths and shifts, corrected for the E2-resonance effect, were used to investigate the nucleon density in the nuclear periphery. The deduced neutron distributions are compared with results of the previously introduced radiochemical method and with Hartree-Fock-Bogoliubov model calculations